Chemical composition, dry matter production and yield dynamics of tropical grasses mixed with perennial forage legumes

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1 Tropical Grasslands (006) Volume 40, Chemical composition, dry matter production and yield dynamics of tropical grasses mixed with perennial forage legumes Z. TESSEMA AND R.M.T. BAARS Department of Animal Sciences, Alemaya University, Dire Dawa, Ethiopia Larenstein University for Professional Education, Deventer, The Netherlands Abstract To evaluate the dry matter (DM) yield, relative yield, relative total yield, aggressivity index and relative crowding coefficient, stability and chemical composition of grass-legume mixtures, an experiment was conducted in a randomised complete block design with 3 replications. Chloris gayana, Panicum coloratum, Phalaris aquatica, Desmodium uncinatum and Medicago sativa were planted as both pure stands and grasslegume mixtures in the first week of June 996 to give treatments, and observations continued for 3 years. The highest DM yields were obtained from pure Chloris and Panicum plus 4 mixtures: Chloris-Medicago, Chloris-Desmodium, Panicum-Medicago and Panicum-Desmodium mixtures, with 3 5 t/ha per year in Years and 3. Phalaris and its mixtures yielded well in the first year, but declined steadily after that. The relative total yields of mixtures were greater than unity, indicating that the DM yields of mixtures were higher than those of an average of the pure stands. The mean relative crowding coefficient values of Panicum and Chloris in mixtures with Medicago and Desmodium were high, indicating that they produced high DM yields in the grass-legume combinations. Both Chloris and Panicum showed positive aggressivity index values in the grasslegume mixtures, indicating that both were more competitive than the legumes, with Panicum more competitive than Chloris. Pure legume stands and grass-legume mixtures produced forage with higher CP and lower fibre concentrations than Correspondence: Tessema Zewdu, Department of Animal Sciences, Alemaya University, PO Box 88, Alemaya, Ethiopia. tessemaz@yahoo.com pure stands of grass. Mixtures of Chloris and Panicum with Desmodium and Medicago seem quite productive and these grass-legume mixtures should be evaluated under smallholder farm conditions. Issues to be addressed are animal performance under both cut-and-carry systems and grazing conditions as well as the stability of the mixtures over time. Introduction Livestock play a crucial role in peasant farming systems and general agriculture in Ethiopia. Currently, productivity per animal is very low, and the contribution of the livestock sector to the overall economy is much lower than expected. A major constraint to the livestock industry is feed inadequacy. Both under-nutrition and malnutrition are major problems for the greater part of the country and for most of the time (Lulseged 985). Livestock depend on natural pastures and crop residues and both quantity and quality of these feedstuffs are too low to sustain satisfactory levels of animal production. The development of grass-legume pastures is one of the recognised strategies in other countries for enhancing both quantity and quality of feed resources. Forage quality and seasonal distribution of biomass of grass-legume pastures have proved superior to those of grasses or legumes grown alone (Daniel 990; Minson 990). Since the emphasis should be on low-input production systems, such a forage strategy could play an important role in improving both the quality and yield of forage, without the addition of organic and/or inorganic fertiliser. In addition, grass-legume mixtures provide advantages over pure stands by reducing the incidence of bloat from legumes, the effects of diseases and insect pests and the level of soil erosion (Mannteje 984; Lulseged 985; Minson 990). The adaptability, yield and persistence of promising perennial grasses and legumes in pure

2 Performance of grass-legume mixtures 5 stands in north-western Ethiopia have been evaluated (AARC 995), and there was a need to extend this work to grass-legume mixtures. The objectives of the experiment reported in this paper were to: evaluate the dry matter (DM) yield, relative yield, relative total yield, aggressivity index, relative crowding coefficient and stability of a number of grass-legume mixtures; and determine the chemical composition of the mixtures compared with pure stands of grasses or legumes. Materials and methods Location of the study A grass-legume experiment was conducted during at Adet Agricultural Research Centre, north-west Ethiopia ( 7 N, E; elevation 40 masl), 445 km from Addis Ababa on nitosols, which were the typical soil type in the region. The 0 40 cm layer of the soil before fertiliser application had a ph of , total N of %, available phosphorus of 0.66 ppm, electrical conductivity of Mmhos/cm and organic carbon of.5 4.4%. The mean annual rainfall of the area ( ) is 85 mm (range mm) with 09 rainy days per year. The rainy season extends from May to October with a peak in June August. The average annual minimum and maximum air temperatures are 8.8 and 5.4 C, respectively (AARC 995; Tessema et al. 00). Experimental design and management The study was conducted using a randomised complete block design with 3 replications. A total of treatments (3 grasses, legumes, 6 grasslegume mixtures) were used. The grass species were: Chloris gayana cv. Masaba, Panicum coloratum and Phalaris aquatica cv. Sirosa; and the perennial forage legumes were: Desmodium uncinatum cv. Silverleaf and Medicago sativa cv. Hairy Peruvian. The 6 grass-legume treatments considered all combinations of the legumes and 3 grasses. Inclusion of pure grass and legume stands allowed relative yields in mixtures to be assessed. The forages were planted in the first week of June 996. Seeds of grasses and legumes were weighed, thoroughly mixed and broadcast on 3 m 5 m plots. An overall seeding rate of 0 kg/ha was used throughout. In the mixtures, sowing rate was 5 kg/ha for both grass and legume. Diammonium phosphate was applied at planting at 00 kg/ha on all treatments. Nitrogen fertiliser (50 kg/ha N) was applied annually as urea for the grass-only treatments according to the recommendation of Bogdan (977) and IAR (988). Data collection and statistical analyses The grass, legume and weed proportions of the mixtures were recorded annually during the crop seasons. All pure stands of grasses and legumes were harvested at about 5 cm above ground level at 50% flowering of the legumes and 00% flowering of the grass, while the grasslegume mixtures were harvested when at least one component of the mixture had reached 50% flowering, based on continuous visual observation of each component in the mixture. One harvest was obtained in 996, while 3 and 4 harvests were obtained in 997 and 998, respectively, for all treatments. The relative DM yields (RY) of the components in the mixtures were calculated using the equations of De Wit (960): RY G = DMY GL /DMY GG () RY L = DMY LG /DMY LL () where DMY GG is the dry matter yield of any perennial grass G as a monoculture; DMY LL is the dry matter yield of any perennial legume L as a monoculture; DMY GL is the dry matter yield of any perennial grass component G grown in mixture with any perennial legume L ; and DMY LG is the dry matter yield of any perennial legume component L grown in mixture with any perennial grass G. Relative total yield (RTY) was calculated according to the formula of De Wit (960): RTY GL = (DMY GL /DMY GG ) + (DMY LG /DMY LL ) (3) The relative crowding coefficient (RCC) of the perennial grass-legume mixtures was calculated to determine the competitive ability of the grass and legume in the mixture to measure whether that component has produced less or more dry matter yield than expected in a 50:50 perennial grasslegume mixture according to De Wit (960): RCC GL = DMY GL / (DMY GG DMY GL ) (4) RCC LG = DMY LG /(DMY LL DMY LG ) (5)

3 5 Z. Tessema and R.M.T. Baars The dominance or aggressive ability of the perennial grasses against the perennial legumes in 50:50 mixtures was described by calculating the aggressivity index (AI) as indicated by McGilchrist (965) and McGilchrist and Trenbath (97): AI GL = DMY GL /(DMY GG 0.5) DMY LG /(DMY LL 0.5) (6) All treatments were harvested about 5 0cm above the ground, fresh forage was weighed in the field and duplicate samples (50 g) were taken after thorough mixing for DM analysis by oven drying at 65 C for 7h until constant weight was obtained. Dried samples were ground to pass a mm sieve and stored in airtight containers prior to chemical analysis. Total ash was determined by igniting at 550 C overnight, DM by drying at 05 C and N by auto-analyser (Chemlab 984). Crude protein (CP) was calculated as N 6.5. NDF, acid detergent fibre (ADF) and acid detergent lignin (ADL) were determined according to Goering and van Soest (970). Hemicellulose and cellulose were calculated as NDF ADF and ADF ADL, respectively. All chemical analyses were done in duplicate to increase the precision of the results. Pure stands of Phalaris aquatica and its mixture with legumes were not included in the chemical analyses due to their lower DM yield compared with other treatments throughout the study. Analysis of variance was carried out using the General Linear Model procedures (SAS 998) for DMY, RY, RTY, RCC, AI and the nutrient concentrations applied to a randomised complete block design. Mean separation was tested using the least significant difference technique at P = Results Monthly rainfall, number of rainy days and minimum and maximum air temperatures during the study are presented in Table. The rainy season during the study extended from May to October with a peak during July September, with total rainfall above average in all years. In pure stands, DM yields of grasses were similar in the first year and exceeded those of legumes (Table ). While yields of Chloris and Panicum increased in the second year then plateaued, yields of Phalaris progressively declined. DM yields of pure legume stands increased progressively during the study, so that yields in Year 3 were only slightly below those of Chloris and Panicum. As for the other treatments, DM yields of the mixed pastures increased with time. This effect was less pronounced in mixtures with Phalaris (Table ) so that yields of mixtures with Chloris and Panicum exceeded those with Phalaris in 997 and 998 (P< 0.05). In Year 3, yields of pure stands of Chloris and Panicum averaged. t/ha while their mixtures with the legumes produced 5.4 t/ha. Medicago regrew rapidly after harvest, while Desmodium was slow. The mean RYs and RCCs of the components of the mixtures throughout the experiment are presented in Table 3. The mean RYs of both legumes and grasses were less than unity indicating that the DM yields of both grasses and legumes in the mixtures were less than those for pure stands. However, all exceeded 0.5 with higher values for grasses than for legumes. The 3-year mean RTYs of all grass-legume mixtures in the experimental periods were greater than one (range:.9.48), Table. Monthly total rainfall (mm) and number of rainy days during the study, Months Rainfall (mm) Temperature ( C) Maximum Minimum May 74 (7) 47 (9) 90 (0) June 05 () 59 (8) 0 () July 34 (8) 40 (8) 385 (7) August 76 (8) 95 (7) 35 (4) September 76 (3) 38 (4) 55 (0) October 0 (7) 65 (0) 53 (8) November 05 (0) 45 () 39 (7) December () 8 (3) Total 398 (36) 450 (47) 547 (30) Numbers in parentheses indicate the number of rainy days.

4 Performance of grass-legume mixtures 53 Table. Mean dry matter yield of 3 perennial grasses, perennial legumes and their mixtures in 996, 997 and 998. Treatments Mean (996 98) Grass Legume Total Grass Legume Weed Total Grass Legume Weed Total (t/ha) Phalaris aquatica ab de d 4.78d Chloris gayana a a 3.08 nil 3.08abc.68a Panicum coloratum abc ab.34 nil.34abc 9.77ab Medicago sativa.60.60c cd abc 6.3cd Desmodium uncinatum bc cd bcd 6.0cd P. aquatica + M. sativa ab cd bc 7.75bc P. aquatica + D. uncinatum a cd nil 6.04cd 6.6cd C. gayana + M. sativa ab ab a.a C. gayana + D. uncinatum a a ab.7a P. coloratum + M. sativa ab a nil 5.6ab.33a P. coloratum + D. uncinatum ab a nil 3.79abc 0.95ab Mean s. e Within columns for totals and means, values followed by different letters are significantly different (P < 0.05). Table 3. Relative yields and relative crowding coefficients of Panicum coloratum, Chloris gayana and Phalaris aquatica grown in mixtures with Desmodium uncinatum and Medicago sativa. Combinations Relative yield Relative crowding coefficient Mean Mean Phalaris with Medicago Phalaris with Desmodium Chloris with Medicago Chloris with Desmodium Panicum with Medicago Panicum with Desmodium Medicago with Phalaris Medicago with Chloris Medicago with Panicum Desmodium with Phalaris Desmodium with Chloris Desmodium with Panicum Relative yield = Yield when grown in a mixture relative to yield as pure stand. Relative crowding coefficient = Yield when grown in a mixture as a proportion of (yield in pure stand less yield in mixture). demonstrating higher yields in mixtures than the average of the pure stands (Table 4). The RCCs of the perennial grass-legume mixtures were calculated to determine the competitive ability of the grass and legume in the mixtures, that is, whether that component has produced less or more dry matter yield than expected in a 50:50 perennial grass-legume mixture. Both grasses and legumes produced RCCs in excess of unity indicating that all yielded better than expected in a 50:50 mixture. However, RCCs from grasses generally exceeded those for legumes indicating their superior ability to perform under these conditions. This dominance by grasses is supported by AI values. The mean AI values of the perennial grasses in mixtures with perennial legumes (Table 4) ranged from to +0.8, indicating that the grasses were dominant in the mixtures and contributed more to the increased DM yield of the mixed sward. Crude protein concentrations were higher in legumes (mean 3.4%) than in grasses (8.%), with the mixed pasture being intermediate (Table 5). Differences in other chemical concentrations were generally not great.

5 54 Z. Tessema and R.M.T. Baars Table 4. Relative total yield and aggressivity index of Panicum coloratum, Chloris gayana and Phalaris aquatica grown in mixtures with Desmodium uncinatum and Medicago sativa. Combinations Relative total yield Aggressivity index Mean Mean Phalaris with Medicago Phalaris with Desmodium Chloris with Medicago Chloris with Desmodium Panicum with Medicago Panicum with Desmodium Relative yield of grass plus relative yield of legume. Aggressivity index = (Actual yield of component/expected yield of component) (Actual yield of other component/expected yield of other component). Table 5. Chemical composition of Panicum coloratum, Chloris gayana and Phalaris aquatica grown in mixtures with Desmodium uncinatum and Medicago sativa. Treatments Chemical concentrations (% DM basis) DM OM TA CP NDF ADF ADL Cellu HC SS BS Chloris gayana bc 55.7ab 33.8ab a.9ab.7.5 Panicum coloratum c 58.8a 34.4a a 4.4a.4.57 Medicago sativa a 5.abc 36.a a 4.9bcd Desmodium uncinatum a 46.5c 35.4a a.d C. gayana + M. sativa abc 46.9c 3.b b 5.8bcd C. gayana + D. uncinatum bc 48.9bc 35.6a a 3.3cd P. coloratum + M. sativa abc 46.c 34.7a a.6d P. coloratum + D. uncinatum abc 54.9ab 34.7a a 0.abc Mean s. e DM = Dry matter; OM = Organic matter; TA = Total ash; CP = Crude protein; NDF = Neutral detergent fibre; ADF = Acid detergent fibre; ADL = Acid detergent lignin; Cellu = Cellulose; HC = Hemicelluloses; SS = Sand silica; BS = Biogenic silica. Within columns, values followed by different letters differ significantly (P < 0.05). Discussion In the current study, Phalaris performed poorly and will not be discussed further. When referring to grasses in this discussion, Chloris and Panicum will be the grasses under consideration. Pure stands of Chloris and Panicum produced superior dry matter yields to pure legumes in the first two years but differences were no longer significant in the third year. This highlights the ability of legumes to produce significant DM yields of forage once properly established and warns against drawing conclusions after shortterm studies. It is normal for grass stands to establish more rapidly than legumes and to outyield them in the early stages. This is in part due to the ability of grasses to utilise the N, which is released following cultivation. Interestingly, yields of pure grass were not significantly different from those of the mixed pastures throughout the study, although yields of the mixtures in the third year exceeded those of the pure grass (5.4 vs. t/ha). However, it must be acknowledged that the pure grass stands received annual applications of 50 kg/ha N. As well as contributing dry matter directly within the combination, legumes contribute indirectly through rhizobial fixation of atmospheric nitrogen. The amount of N fixed in white clovergrass pastures has been estimated to be as high as 680 kg N/ha/yr, although kg N/ha/yr is more commonly achieved (Mu and McGechan 999). In the western parts of Ethiopia, Lemma et al. (99) reported that pure Chloris produced higher DM yields than Chloris-legume mixtures in the second and third years after establishment. On the other hand, Daniel (990) and Minson (990) found that legume-grass pastures outyielded pure grass pastures. The dry matter yield from natural pasture oversown with Stylosanthes

6 Performance of grass-legume mixtures 55 guianensis exceeded that of the natural pasture in subtropical areas of Ethiopia (Lemma et al. 993). The grasses and Medicago grew rapidly but Desmodium grew slowly in the rainy season during the study period. As the growth patterns of legumes and grasses are variable, selection of the correct grass-legume combination could make sustained animal feed available year round. The DM production potential of the grasslegume mixture in the present study was similar to those reported by other research works (Morrison 984; Lemma et al. 99). Principally, when a pure grass pasture is grown, there is a tie-up of N in below-ground parts, with resultant reduction in yield through N deficiency. This N can be released periodically by renovation, but growing a legume with the grass can provide N from biological fixation of atmospheric N to correct the deficiency. Morrison (984) indicated that legumes such as Medicago and clovers increase the yield of grasses when grown in combination with them. The 3-year mean RTYs for the mixtures ranged from.9.48, indicating a yield advantage of 9 48 percent over that from an average of pure stands of the different species (grass and legume). This may indicate N contribution to the grass through nitrogen fixation by the legumes and its transfer from the legume component to the grass. The results of the RY and RTY in the current study were in agreement with findings of Daniel (990) and Diriba (00) for Chloris-Medicago and Panicum-Stylosanthes mixtures, respectively. This situation could be attributed to the efficient utilisation of plant growth factors by species in the mixture due to either temporal or spatial differences of their demands. The AI values for Chloris mixed with Desmodium and Chloris mixed with Medicago were and +0.3, respectively, indicating that Chloris and Desmodium were quite compatible but that Chloris was more competitive with Medicago. AI values for Panicum in mixtures with the two legumes were much higher than those for Chloris but again were much higher in combination with Medicago than with Desmodium. All pastures contained high levels of crude protein at harvest, which far exceeded the level below which feed intake is restricted, and also exceeded the level required for satisfactory production of 5.0% (Norton 98; van Soest 98). However, legumes had higher protein concentrations than grasses, which agrees with other published studies (Lemma et al. 99; Getnet 999). A key advantage of perennial legumes in a mixed pasture is their higher protein value at any given time than the accompanying grass species (Lemma et al. 99). Crowder and Chedda (98) and Daniel (990) indicated that this higher crude protein concentration in legumes in a mixed pasture could extend the productive period for animals further into the dry season. This provided an additional advantage over and above the contribution of nitrogen to the companion grasses. Pure grasses had higher NDF, ADF, cellulose and hemicellulose values than pure legumes and their mixtures in the present study. The threshold level of NDF in tropical grass beyond which DM intake of cattle is affected is 600 g/kg (Meissner et al. 99), and all grasses and legumes and their mixtures had lower values (range g/kg). In the present study, the cellulose (range 48 9 g/kg) and hemicellulose (range 40 g/kg) concentrations in all grasses and legumes, and their mixtures were lower than those quoted for most tropical grasses (39 and 354 g/kg, respectively; Moore and Hatfield 994). Since animals can select a better quality diet than feed on offer when allowed to selectively graze, these issues should not be a major problem. Conclusions Chloris and Panicum have more potential than Phalaris in this environment. Grass-legume mixtures were at least as productive as pure grass stands, which received 50 kg/ha N. All combinations of Chloris and Panicum with Desmodium and Medicago were highly productive and could be considered for cut-and-carry feeding systems and for hay production under smallholder farm conditions. However, mixed pastures can be more difficult to manage than single species pastures and the long-term stability of these mixed pastures under regular cutting or grazing cannot be determined from results of this study. Acknowledgements The authors acknowledge the Adet Agricultural Research Centre (AARC), for financing the research. We thank Mr Demekash Asregid, staff member of the Animal Feeds and Nutrition Research Division of AARC, for his assistance during the study.

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